1. The Field of the Present Invention
The present invention relates generally to the field of automotive instrumentation, and more specifically to an apparatus and method for accurately driving any mechanical cable-driven speedometer instrument, as is typically found on older vehicles.
2. General Background
For the purposes of this application, the terms “processor”, “microprocessor”, “controller”, “microcontroller” or equivalent terminology are meant to be synonymous unless otherwise stated. Likewise, the terms “automobile”, “truck”, “vehicle”, or equivalent terminology are meant to be interchangeable unless otherwise stated. References made in the English measurement system are hereafter assumed to include their metric equivalent values and vice versa.
One of the principal instruments in all motor vehicles is the speedometer. It is universally recognized that knowing how fast one is driving is an important aspect of operating the vehicle. To that end, the mechanical speedometer, a wonderfully clever instrument, was invented. Patent office records indicate the first speedometer patent, patent number 1,335,833, was issued in 1920.
It should be noted that speedometer instruments typically display two different measurements: road speed displayed on the speedometer proper and total distance driven displayed on the odometer. Though housed in the same housing, the odometer and the speedometer are actually two separate and distinct measuring devices. The odometer measures distance traveled, and the speedometer measures the rate of speed at which the distance is currently being traversed. To avoid confusion, when speaking of the speedometer instrument as a whole, including both the speed-measuring “speedometer” and the distance measuring “odometer”, the phrase “speedometer instrument” will be used. Either measuring device, taken independently of one another, will be referenced by “speedometer” or “odometer” as appropriate, unless prefaced by a descriptive phrase such as “mechanical speedometer” or “electronic speedometer”, in which case the entire speedometer instrument is meant, not just its speed-measuring portion.
In a properly functioning speedometer instrument, the speedometer and odometer are calibrated such that the two measuring devices concur and cohere with one another and with the actual road speed of, and distance traversed by the vehicle. An example will help to clarify: if the vehicle is in fact driven at a constant 60 mph for one hour, the properly calibrated speedometer should read a constant 60 mph during that one hour of travel time, and at the end of the hour the odometer should show that the total mileage driven has indeed incremented by 60 miles; likewise, if the odometer registers an increment of 1 mile, and if that mile happened over the course of one minute, and if the vehicle in question has been driven at a constant speed for one minute, then the speed should have displayed as a constant 60 mph for the duration of the minute. As 60 mph is equivalent to one mile per minute the vehicle will therefore have actually traveled exactly one mile during the one minute.
In addition to being physically located in the same housing, the speedometer and the odometer typically also operate off of the same input. “Input” for speedometer instruments reduces in the end to information about the rotation rate of the road wheel, whether detected directly, or as is more usually the case, detected indirectly by means of a physical or other (e.g. electronic) connection to some part of the drive train that rotates at a rate proportional to the rotation rate of the road wheel.
For the mechanical speedometers, the “input device”, that is, the means by which the speedometer instrument receives information about the rotation rate of the road wheels, is the speedometer cable, which is a mechanically rotating sheathed cable. The speedometer cable physically attaches to and rotates an input shaft on the back of the speedometer instrument. The other end of the speedometer cable physically attaches to and is rotated by a set of gears called the “drive gear set” or “gear set”, which is usually (but not necessarily) physically located in the vehicle transmission. The drive gear set is operatively (but not usually directly physically) connected to the road wheels, so that the gears in the drive gear set rotate at a rotation rate proportionate to the rotation rate of the road wheels. The relative size of the gears in the gear set (otherwise referred to as the “speedometer drive gear ratio”) determines the rotation rate of the speedometer cable relative to the rotation rate of the road wheels.
With mechanical speedometers, typically, the odometer portion of the instrument is mechanically connected to the instruments rotating input shaft. This is generally accomplished by the use of small gears inside the speedometer instrument. These gears are driven by the input shaft, which in turn is driven by the speedometer cable. Whereas the “odometer” is mechanically coupled, the “speedometer” portion of the instrument, is typically magnetically coupled to the same rotating input shaft. The effective amount of coupling is determined by a spring that counter-balances the driven magnet inside the instrument.
By design, all mechanical speedometers require a specific number of revolutions of the input shaft (via the speedometer cable) to register an increment of one mile on the speedometer instrument's odometer (assuming the instrument in question was designed to display British Imperial or U.S. measurement units). This odometer-specific number of revolutions of the input shaft (via the speedometer cable) is known as the “odometer constant”, which for the mechanical speedometer is expressed in “revolutions per unit distance”, specifically, for instruments using British Imperial or U.S. measurement units, “revolutions per mile”.
The “speedometer proper,” that is, the speed-measuring device inside the speedometer instrument, is calibrated such that the speed displayed on the speedometer instrument concurs and coheres with incremental changes in the odometer reading. Specifically, it is calibrated such that when the speedometer reads 60 mph, the speedometer cable is driving the speedometer instrument input shaft at a rotation rate per minute numerically equal to the odometer constant. This calibration factor is expressed as the “speedometer constant”, which, to turn the definition around, is the number of revolutions per minute of the input shaft (and hence the speedometer cable) at which the speedometer indicates a speed of 60 mph.
Although the odometer constant is measured in revolutions per mile (distance) and the speedometer constant is measured in revolutions per minute (time), the odometer and speedometer constants will be numerically equal in any given speedometer instrument if the instrument is working properly. It is known and understood that it takes a specific number of revolutions of any given speedometer instrument input to cause the odometer to register a one-mile change in distance. Since this is a “distance only” measurement, the time it takes to accomplish it is immaterial. It is also known that the speedometer portion of the instrument that measures miles/hour (distance per unit time) is time dependent. Since both are simultaneously measuring the rotation of the same input shaft, we can see that the action of the two parts of the instrument must correlate if it is deemed “calibrated”. For example, the same number of input revolutions that causes the odometer to register a one-mile change must happen in exactly one minute in order to cause the speedometer to register 60 mph (1 mile/minute). More or less time will result in a higher or lower indication on the speedometer. Modifications of this brief explanation of speedometer and odometer constants for instruments manufactured using metric or other measurement units are straightforward matters of unit conversions.
As the speedometer and odometer constants are numerically equal, the term “speedometer constant” will be used unless a clear distinction needs to be made between “revolutions per minute” and “revolutions per mile”. Tire circumference and the speedometer constant are the two main factors involved when designing a vehicle speed measuring system. The interposing “speedometer cable drive gear set” ratio is chosen such that every rotation of the fitted tire will cause the input to the speedometer to turn the appropriate amount. Thus, neither the tire circumference nor the speedometer constant determines what the other must be. However, the example below is accurate if one assumes a 1:1 “speedometer cable drive gear set” ratio.
An example of the relationship between tire circumference, speedometer constant and speedometer drive gear ratio follows: If the speedometer constant is 1026 rev/mile, the tire must travel one mile every 1026 turns of the speedometer cable, which means that the tire circumference must be 1/1026 mile or .00975 mile, or as more usually measured, 61.75 inches, and hence the tire diameter must be 61.75/“pi” or 19.7 inches. As there are 63,360 inches in one mile, a 61.75-inch circumference tire will rotate 63,360/61.75 or 1026 rotations in one mile. Hence the 1:1 drive gear ratio. Changes to either tire size or speedometer constant are generally compensated for by changes in the speedometer drive gear ratio. In the 1970s a new variation on the speedometer instrument appeared, typically referred to as an “electronic speedometer”, whose pointer was made to move in response to a train of electronic pulses generated by a pulse generator, which pulse generator is usually located inside the transmission of the vehicle. The “electronic speedometer”, of which there are many varieties and which is in use on many current production vehicles, has no need for the mechanical speedometer's rotating cable. The speedometer and odometer constants remain applicable for the electronic speedometer, but the relevant units are impulses per minute and per mile, rather than revolutions per minute and per mile. Another recent innovation in speedometer technology is the digital speedometer. Digital speedometers are electronic speedometers that display speed using an LED (Light Emitting Diode) or LCD (Liquid Crystal Display) instead of a mechanically driven pointer. None of these technologies employ the mechanical speedometer's rotating cable.
Despite the newer variations on speedometer instrumentation, many vehicles on the road today, and even some of the more recent production vehicles, still use the mechanical speedometer with its rotating cable drive. Something as simple as a change in tire size when replacing worn tires will render inaccurate the speed and odometer readings of not just the mechanical speedometer, but also the other types of speedometer outlined above. There exist various ways of addressing this problem for each type of speedometer instrumentation.
Throughout the world many individuals derive great pleasure from restoring, rebuilding, or otherwise customizing motor vehicles. A universal problem facing owners of vehicles with mechanical speedometers is keeping the speedometer instrument readings accurate in the face of vehicle repairs, upkeep, or custom modifications involving the various components of the drive train. Although there are methods to regain the accuracy of the speedometer instrument in any given situation, the means to do so is labor-intensive, time-consuming and generally expensive, involving the services of specialists who maintain facilities and equipment specifically designed for this purpose. Even if these expensive and time-consuming methods are employed, further modifications to the vehicle's drive train components may and likely will again render the speedometer inaccurate; and will require yet another investment of time and money to, yet again, correct the problem. Many people choose to live with speedometers that are mildly to grossly inaccurate simply because of the difficulty and expense in keeping them accurate. The present invention addresses this problem.
Although accurate aftermarket speedometer instruments are available as replacement units, very often vehicle owners find it aesthetically or otherwise desirable to keep the original mechanical instrumentation with their vehicle. The present invention has been created in light of the need to circumvent this problem and other problems highlighted herein.